1. Field of the Invention
The present invention relates to a red phosphorus flame retardant and a nonflammable resinous composition containing the same. In particular, the present invention is directed to a red phosphorus having a special surface configuration which has been produced by a special process and a nonflammable resinous composition containing the red phosphorus which composition is greatly improved in its moisture-resistance, corrosion-resistance and heat resistance. Further, the present invention is directed to the provision of a nonflammable resinous composition which can be easily and safely handled and is highly stable.
2. Description of the Prior Art
Since red phosphorus is useful as a flame retardant for synthetic resins, it has been heretofore used in thermosetting resins and thermoplastic resins to provide various nonflammable resinous compositions which have been extensively utilized in a variety of applications, such as electronic components or parts, electric articles, machines, automobiles, buildings, etc.
However, when red phosphorus is used as it is, the following problems have been encountered because of its lability and sensitivity to heat, friction and shock. Namely,
red phosphorus presents a danger in handling, storing and mixing with resins;
formation of poisonous phosphine gas and oxidation products is caused due to the reaction of red phosphorus with moisture in the air, thereby polluting the working environment and impairing the physical and electrical properties of resinous compositions; and
there are difficulties in preparing a nonflammable composition due to the lack of compatibility with synthetic resins.
For these reasons, various ways of stabilizing the red phosphorus flame retardant with various organic or inorganic substances have been tried in order to overcome the foregoing problems but they have been entirely successful. Accordingly, the use of the red phosphorus flame retardant is restricted to certain fields and it has been difficult to satisfy the requirements for high qualities.
Presently, the red phosphorus flame retardant has been extensively used as a flame retardant for thermosetting resin, particularly epoxy resin, and has been mainly used in insulating cast resinous compositions for use in electronic components for high voltage applications.
However, in recent years, with an increasing trend toward miniaturization and high-voltage application in electric or electronic articles, increasing demand is being directed to electrically insulating materials with a high performance. For such a demand, the requirements for the physical properties of the red phosphorus flame retardant have become more critical and, thus, red phosphorus flame retardants heretofore available can not fully meet the requirements. In other words, the electronic parts or components using, as an insulator, the nonflammable resinous composition containing the conventional red phosphorus flame retardants are subjected to degradation of insulation and corrosion at metallic portions due to deterioration of the used resin with the passing of time, and thereby their properties are impaired. In such circumstances, it has been pointed out that the known nonflammable articles lack durability and lability. Such a lack is considered to be caused mainly due to deterioration of the red phosphorus flame retardant and, thus, improvement for this has been required. The deterioration of the red phosphorus flame retardant has been considered to be due to the formation of phosphine and corrosive oxidation products resulting from the reaction of the red phosphorus with a small amount of moisture and, as a method of stabilizing the known red phosphorus flame retardants, their powders are coated with various substances so as to be screened from the contact with moisture. However, actually, such a known method itself has limitations and, thus, can not meet the requirements for resinous materials intended to use in high performance electronic components in which an extremely high resistance to moisture and corrosion is required.
As an alternative method to render the insulating cast resin nonflammable for the high voltage applications, organic halide flame retardants have been practically used either singly or in combination with antimony trioxide in some cases, because they have good moisture resistance and corrosion resistance as compared to the foregoing red phosphorus. However, these known halide flame retardants, in addition to the inherent disadvantage that they evolve a large quantity of poisonous gases when burning cause serious deterioration of the electrical properties of the resins because of the use of them is required in large amounts. Further, since the halide flame retardants are expensive, the production cost is increased.
In contrast to this, red phosphorus is considered as a hopeful flame retardant material meeting the requirements, such as safety and minimization of environmental pollution, because evolution of poisonous gases and smoking when burning are slight as compared with the organic halides. Further, since it exhibits a very high flame-retarding ability in a small amount, the use of it not only reduces detrimental effects on the physical properties of the resins, but also is advantageous from the point of cost. Under such circumstances, there is a growing demand for improvements in the heat resistance and moisture resistance of flame retardants of red phosphorus and more stabilized red phosphorus flame retardants are awaited.
Thermoplastic resins have been extensively used in various fields, such as electric articles, machines, automobiles and buildings, because of their superior physical and chemical properties. Generally, thermoplastic resins are subjected to mixing and molding operations at relatively high temperatures in comparison with thermosetting resins and, thus, red phosphorus flame retardant has not so often been used in the resins because of the lack of thermal stability. As other known flame retardants, organic halides, organic phosphorus compounds, antimony trioxide, etc., have been used practically either singly or combinations thereof in thermoplastic resins. However, these known flame retardants have, for example, the disadvantages that they present problems in safety and stability or cause serious deterioration of the physical properties of the resins. Recently, with an increasing demand for much higher quality in all industrial fields, the requirements for thermoplastic resins have also become more strict. For example, with respect to nonflammability contemplated by the present invention, with increasing public demand for safety, a further higher technique has been required not only for obtaining a higher burning resistance but also for securing safety in working and burning and stability. However, most of these retardants can not meet such a requirement. For example, thermoplastic resins are subjected to forming operations at relatively high temperatures and, during such a high temperature operation, the organic halide flame retardant yields corrosive thermal decomposition products or hydrolysis products, thereby damaging the metal mold. Further, after molding, bleed-out occurs at the surfaces of the resulting molded articles and the surface appearance and the electrical properties of the articles are impaired. Further, the organic halide flame retardant should be added in large amounts to impart an enough burning resistance to the resulting products but such a large amount of addition not only adversely affects the mechanical properties, such as tensile strength, folding endurance or impact resistance, but also results in increased production cost. In recent years, as the most serious problems associated with the use of organic halide flame retardants in thermoplastic resins, particular attention has been given to the problems caused by a large amount of smoke or toxic gas generated when burning. With an increasing demand for safety from burning in the use of synthetic resins, the additives like organic halides, which may cause evolution of a large quantity of gas pollutants when burning, have been gradually limited from the viewpoints of personal safety and maintenance of equipments or tools. Antimony trioxide has been usually employed as a flame-retarding assistant for, the organic halide flame retardants, but it not only exhibits detrimental effects on the physical properties of the used resins, particularly with regard to the reduction of tensile strength and impact resistance, but also presents problems or troubles in ensuring the safety of working environments because of its toxicity. Further, it has known that most organic phosphorus compounds themselves act as a plasticizer and, therefore, cause an unfavorable reduction in the heat-resistance and mechanical properties of resins. Also, the organic phosphorus compounds increase the water absorbing property of the nonflammable resinous article, thereby leading to an unfavorable deformation of the article.
In contrast to this, red phosphorus exhibits a very high flame-retarding ability in a small amount and evolution of poisonous gases and smoking are slight as compared to the halide type flame retardant. Therefore, red phosphorus is considered as a hopeful flame retardant material which is safe from burning and minimizes environmental pollution problems. Under such circumstances, the foregoing methods of stabilizing red phosphorus powder by coating have been tried to improve the heat resistance of the red phosphorus flame retardant used in thermoplastic resins, but they have not been successful. Therefore, there is a growing demand for a red phosphorus flame retardant which is stable and safe in working and burning.
In response to such a demand, the present inventors have made many studies on the foregoing problems, such as moisture resistance, corrosion resistance and heat resistance of red phosphorus as a flame retardant, and consider that there are limitations in the conventional method for surface treating red phosphorus powder. On the base of such consideration, the inventors have carefully studied the properties in question from a different angle and, as a result, found that the red phosphorus powder obtained from a novel process different from any prior art have a special configuration and are entirely different in their surface states and physical properties from those obtained from the prior art. The novel red phosphorus has a very high stability and may be employed as a flame retardant as it is. However, such a novel type of red phosphorus has been found to be considerably stabilized by a surface modifying treatment and, thereby, be very useful as a flame retardant for resin compositions. The present inventions have been arrived based on the above findings wherein the above problems with respect to moisture resistance, corrosion resistance and heat resistance can be overcome.